Conventionally, it has been a general practice to detect a reception signal by means of the quadrature detection. The conventional quadrature detector is described in Patent Document 1 (Japanese Laid-Open Patent Publication (Kokai) No. H10-23092 (FIG. 2)), for example. FIG. 7 is a block diagram showing the prior art quadrature detector.
A signal under test output from a signal-under-test source 141 is supplied to multipliers 147 and 148. A reference signal output from a reference signal source 142 is supplied to the multiplier 147, and is simultaneously supplied to the multiplier 148 via a 90-degree phase shifter 145. The multipliers 147 and 148 respectively multiply the supplied signals, and output multiplied results to low-pass filters 151 and 152. Signals after passing the low-pass filters 151 and 152 are respectively referred to as I and Q signals.
The I and Q signals obtained by means of the quadrature detection are processed by a CPU (Central Processing Unit), and the amplitude and phase of the signal under test are then obtained. Moreover, the signal under test output from the signal-under-test source 141 may undergo an A/D conversion.
However, it is difficult to increase the speed of the processing by realizing the quadrature detector described as hardware. If an NCO (Numerical Controlled Oscillator) is employed as the reference signal source 142, for example, the reference signal source 142 has to carry a large sine table. Moreover, if the low-pass filters 151 and 152 are implemented by logic circuits, the amount of logical operations increases. Further, if the CPU receives and processes the I and Q signals, it is necessary to decrease a generation speed of the I and Q signals down to a processing speed of the CPU by means of decimation. If the decimation is carried out, there occurs an aliasing of the signals due to the decimation. It is thus necessary to employ a decimation filter to prevent the aliasing of the signals.
An object of the present invention is to carry out a fast detection of a reception signal.